I didn't get into this show until it was in its 5th season, but I caught up quickly thanks to my brother's DVD sets.
I thought the show had a different feel during it's 1st season, as most shows do. But Frank was that missing ingredient that made the show one of the funniest I have ever seen!
Other people have brought up "The Gang Solves the Gas Crisis." That one is way up there for me. It's one I keep going back to. I tend to watch my favorite episodes many, many times. That episode is actually the first I'd ever seen of the show. I was flipping around and caught a rerun of it. The only part I saw was Dee showing Dennis the newspaper about how their dad was going to give all of his money away. Dee, very nonchalantly, said that her original plan to get that money was to eliminate everybody who had a right to it, which included Dennis. Dennis, of course, questions that plan, which would have included him being killed, and Dee keeps trying to change the subject. At the time I just remember thinking that I'd never seen comedy like that in a sitcom. I put the idea of watching it in the back of my head and changed the channel.
Months later my brother shows me "Who Pooped the Bed?" I could not stop laughing at this episode. I not ashamed. I admit it. I enjoy poop jokes. But what really killed me about this episode was Dee's storyline. Her desperate attempts to have a Sex and the City lifestyle all backfire on her. Artemis saying to those guys at the club, "I have a bleached asshole." The Waitress getting plastered at the shoe store and finally Dee's attempt to shoplift a pair of $700 dollar shoes. I just about died when she tried to run in those heels and fell headfirst into a car door. I heard Kaitlin Olson insisted on doing the stunt herself and she looked like she was in pain. You can see the dent she made in the car door clearly as she's writhing on the ground.
After that I started watching at the beginning of Season 5 after getting caught up.
But my favorite episode has to be "The Gang Gives Back" from Season 2. This was an early episode that truly showed just how self-involved these guys are. From Charlie's attempts to get closer to the Waitress using his alcoholism, Dennis vs. Mac and Dee and the horrific ways they tried to cheat at the Pee Wee basketball game to Frank's Asian gambling buddies. The basketball game is probably the funniest scene in the history of this show. Charlie, completely wasted, as the ref, Frank fixing the game, Mac giving his team Red Bull ("Those are the vitamins ripping out the insides of your stomach."
), and the violence. All at a children's basketball game.
I love this show and I hope it never ends.